1. Field of the Invention
The present invention relates to a low-permeability, multilayer plastic pipe for carrying liquid or gaseous media.
2. Description of the Background
Multilayer plastic pipes having barrier properties with respect to the medium to be carried are known and are described in a large number of patent applications. However, the question which constantly arises is how sufficiently good adhesion can be produced between the polymer to be used for the barrier layer and the polymer which provides the mechanical properties? In particular when fluoropolymers, which are very good barriers with respect to fuels containing alcohols, are used, the problem is a large one which requires complex and, therefore, expensive chemical development work.
Attempts have also been made, for certain applications, to embed the barrier polymer in the carrier material in the form of impermeable sheet-like lamellae, which increase the diffusion path length and thus increase the barrier effect (JP-A-140 588/92, U.S. Pat. No. 4,410,482, German Offenlegungsschrift 43 15 177). However, production of the lamellae requires biaxial orientation, which does occur in blow molding, but not in pipe extrusion. Thus, biaxial orientation utilized in blow-molded tanks cannot be used in pipe extrusion, which only permits the production of fibrils which are unidirectionally oriented in the flow direction of the pipe and do not give the desired barrier effect.
It is also known that in pipes which have been provided with barrier layers by coextrusion, adhesion can become lost after relatively long contact with aggressive media. This is regarded as particularly critical if it is the inner layer which becomes detached, since under certain operating conditions a detached inner layer can partially constrict the pipe cross section in an unacceptable manner and so impair function.
Attempts have, therefore, been made to connect the barrier layer and the outer sheathing by means of mechanical locking (U.S. Pat. No. 5,437,311; DE 42 38 696). However, the tooling necessary for this is difficult to produce and unreliable in operation. In addition, the undercuts which are created function as notches and cause failure of the pipe under sudden load. A need, therefore, continues to exist for improvement in the bonding of a barrier layer polymer to a sheathing polymer in the construction of a multilayer pipe.
Accordingly, one object of the present invention is to provide improved bonding between the barrier layer and the other layers of a multilayer pipe which avoids the disadvantages described.
Briefly, this object and other objects of the present invention as hereinafter will become more readily apparent can be attained by a multilayer plastic pipe comprising at least three layers comprised of at least two different polymers, of a polymer A (barrier layer polymer), which is a particularly good barrier against permeation of the medium to be carried, embedded in a polymer B (sheathing polymer) in the form of at least one layer which is interrupted at least once by a fillet of polymer B.